The field of the invention is in the high frequency radio receiver art and more particularly that of radar receivers having frequency measuring capability.
The operation of prior art Instantaneous Frequency Measurement (IFM) receivers to receive radar pulses can be, and is, frequently foiled by the presence of a continuous wave (cw) signal of sufficient strength.
The conventional Instantaneous Frequency Measurement (IFM) receiver is a radio frequency (RF) receiver used primarily in electronic warfare (EW). Its basic function is to measure the frequency of pulsed signals radiated from hostile radar. Although some IFM receivers have the capability of measuring pulse amplitude (PA), pulse width (PW), and time of arrival (TOA), this invention is only concerned with the frequency measurement capability of the receiver.
An IFM receiver can have wide (giga hertz) instantaneous frequency bandwidth, and its construction is relatively simple compared with other EW receivers with this capability. However, a major deficiency of an IFM receiver is that it can measure only one signal at a time. When two or more signals arrive at the receiver simultaneously, the receiver may generate erroneous information. If a continuous wave (cw) signal is present at the input of the receiver, any other arriving signal will create a simultaneous signal condition. Therefore, when there is a cw signal at the input of the receiver, the receiver cannot measure a pulsed signal.
The best known prior art is that contained by U.S. Pat. Nos. 3,992,666 to patentees Edwards et al, 3,940,699 to patentee Emgushov, 3,939,411 to patentee James, and 3,465,253 to patentee Rittenbach.